The invention relates to the field of antennas, and more particularly to electronically scanned antennas capable of operating at high frequencies including the millimeter wavelength energy regions.
The small size, narrow beamwidths and high resolution of millimeter wavelength antennas make them desirable for many applications. However, due to the narrow beamwidths associated with these antennas, a large number of beam positions is required to cover the same surveillance volume as lower frequency antennas. This may require thousands of radiating elements with associated connectors, dividers, couplers, transmission lines and where scanning is a required antenna function, phase shifters. Due to the short wavelength of this energy, the elements involved are physically very small and maintaining manufacturing tolerances becomes difficult. At a frequency of 60 GHz and higher, the components are typically extremely small and difficult to accurately, consistently and practically reproduce. Fabricating and assembling these components also pose large cost considerations.
At lower frequencies, individual phase shifters have been employed. One phase shifter for each radiating element is used in a typical antenna, and a phased array may include hundreds or even thousands of such elements spaced one-half wavelength apart, for example. At a frequency such as 60 GHz, the use of individual phase shifters becomes difficult for the reasons discussed above.
A prior technique for a millimeter wavelength antenna is found in R. E. Horn, H. Jacobs, E. Freibergs and K. L. Klohn, "Electronic Modulated Beam-Steerable Silicon Waveguide Array Antenna", IEEE Transactions, MTT, Vol. MTT28, No. 6, June 1980, pp. 647-653. In this technique, a silicon rod with a metallic grate on one surface and distributed PIN diodes on an adjoining surface are stated to be operable near 60 GHz. This technique is apparently limited in usefulness, however, in that relatively high rf losses occur with this structure (page 651); the scan range is relatively small (approximately 10.degree., page 649); the ability to continuously scan the beam is doubtful (page 650); and the technique is complex.
Another technique involves using ferrite phase shifters as radiators. An antenna using this technique is found in U.S. Pat. No. 3,855,597 to Carlise. Ihis antenna uses a partially ferrite loaded, slotted waveguide for electronic scanning. The waveguide is loaded with ferrite sections which coincide with radiating slots in the waveguide. This approach is a modified Reggia-Spencer type phase shifter and retains most of the problems of the Reggia-Spencer approach.
In the Carlise technique as in general in Reggia-Spencer type radiators, the discontinuities between the empty, or dielectric portions of the waveguide and the ferrite portions permit undesirable higher order modes. A holding current is required of the control coils to keep the beam pointing in a given direction and the magnitude of this holding current must be very accurately controlled or the antenna beam will scan off the given direction. This holding current requirement puts a severe drain on the control current power supply. Latching yokes have not been used since the ferrite does not fill the waveguide. The dielectric or air gaps in the magnetic field path present such a large impedance to the magnetic field generation circuit that phase control coils wound around the waveguide adjacent to the radiating slot have been used. The proximity of these coils to the slot can result in the coupling of the radiated rf energy into the control coils thereby causing rf loss and antenna pattern degradation. Since the ferrite is not in contact with a thermally conductive material such as the waveguide, cooling is effected by radiation unless an additional cooling apparatus is attached. Heat dissipation techniques for cooling the ferrite rod, other than radiation only, have entailed physical difficulties. Manufacturing difficulties exist in accurately and consistently assembling the ferrite sections with air or dielectric spacers in between, supporting this ferrite/spacer rod inside the waveguide, and maintaining consistency in the windings between each radiating waveguide aperture.